This invention relates to improvements in the Asphalt Residual Treating (ART.TM.) Process for upgrading hydrocarbon feedstocks contaminated with heavy metals. In particular, the invention relates to improving operation of ART units when the feedstock becomes contaminated with or is likely to become contaminated with halogens such as sodium chloride, resulting in an increase in coke or coke and hydrogen production in excess of levels anticipated on the basis of removal of Conradson Carbon in the feedstock and metals content of the contact material.
The Asphalt Residual Treating (ART) Process is a decarbonizing and demetallization process that has been developed to treat residual stocks and heavy crudes for the removal of contaminants. The process is described in numerous publications, including "The ART Process Offers Increased Refinery Flexibility", R. P. Haseltine et al, presented at the 1983 NPRA Conference in San Francisco. See also U.S. Pat. No. 4,263,128 to Bartholic. The process is a non-catalytic technological innovation in contaminant removal and will typically remove over 95% of the metals, essentially all the asphaltenes and 30% to 50% of the sulfur and nitrogen from residual oil while preserving the hydrogen content of the feedstock. This provides greatly improved cost-effectiveness by producing less unwanted by-products and consuming less energy than competing processes. The ART Process also enables the subsequent conversion step in residual oil processing to be accomplished in conventional downstream catalytic processing units.
The ART Process utilizes a fluidizable solid particulate contact material which selectively vaporizes the valuable, lower molecular weight and high hydrogen content components of the feed. The contact material is substantially catalytically inert and little if any catalytic cracking occurs when the process is carried out under selected conditions of temperature, time and partial pressure. Generally, suitable contact material has a relatively low surface area, e.g., 5 to 20 m.sup.2 /g as measured by the BET method using nitrogen. Heavy metals are deposited on the contact material and removed. High molecular weight asphaltenes also deposit on the contact material, some asphaltenes being converted to lighter products.
The ART process is adapted to be carried out in a continuous heat-balanced manner in a unit consisting primarily of a contactor, a burner and an inventory of recirculating contact material. Chargestock is contacted with particles of hot fluidizable contact material for a short residence time in the contactor. In the contactor, the lighter components of the feed are vaporized; asphaltenes and the high molecular weight compounds, which contain metals, sulfur and nitrogen contaminants, are deposited on the particles of the contact material. The metals invariably include vanadium and nickel. Some of the asphaltenes and high molecular weight compounds are thermally cracked to yield lighter compounds and coke. The metals that are present, as well as some of the sulfur and nitrogen bound in the unvaporized compounds, are retained on particles of contact material. At the exit of the contacting zone, the oil vapors are rapidly separated from the contact material and then immediately quenched to minimize incipient thermal cracking of the products. The particles of contact material, which now contain deposits of metals, sulfur, nitrogen, and carbonaceous material are transferred to the burner where combustible contaminents are oxidized and removed. Regenerated contact material, bearing metals but minimal coke, exits the burner and circulates to the contactor for further removal of contaminants from the charge stock.
In practice, the metals level of contact material in the system is controlled by the addition of fresh contact material and the removal of spent contact material. A high metals level can normally be maintained without detrimentally affecting performance.
Because the contact material is essentially catalytically inert, very little molecular conversion of the light gas oil and lighter fractions takes place. Therefore the hydrogen content of these streams is preserved. In other words, the lighter compounds are selectively vaporized. The molecular conversion which does take place is due to the disproportionation of the heavier, thermo-unstable compounds present in the residual feedstock.
The hydrogen content of the coke deposited on the contact material is typically less than four percent. Coke production is optimally equivalent to 80% of the feedstock Conradson Carbon Residue content. Heat from the combustion of coke is used internally within the ART system. Surplus heat may be recovered as steam or electric power. No coke product is produced. In contrast, delayed and fluid cokers yield a coke product equivalent to 1.3 to 1.7 times the Conradson Carbon residue.
Generally, metals accumulated on the contact material used in the ART process tend to be less active in forming coke than metals accumulated on cracking catalyst. Thus, the ART process is able to operate effectively when accumulated metals are present on the contact material at levels higher than those which are generally tolerable in the operation of FCC units. For example, the process has operated effectively when combined nickel and vanadium content substantially exceeded 2% based on the weight of the contact material. However, during operation of one particular ART unit, coke production began to increase to levels that were considerably higher than would be expected based on the Conradson Carbon content of the feed and metals content of the contact material. Hydrogen production also increased. In other words, it appeared that metals deposited on the circulating inventory of contact material had become activated. In the operation of FCC units, a similar excursion from normal operation may be experienced but, generally, at lower metals levels.